X-ray pictures can be stored in so-called storage phosphors, whereby the X-ray radiation passing through an object, for example a patient, is stored as a latent picture in a storage phosphor layer. In order to read out the latent picture, the storage phosphor layer is irradiated with stimulation light, and so stimulated into emitting emission light. The emission light, the intensity of which corresponds to the picture stored in the storage phosphor layer, is collected by an optical detector and converted into electric signals. The electric signals are further processed, as required, and finally made available for examination, in particular for medical diagnostic purposes, whereby they are displayed in a corresponding display unit, such as a monitor or a printer.
In the patent document U.S. Pat. No. 4,484,073, herein incorporated by reference in its entirety for background information only, a device and a method are described whereby a laser beam is deflected by means of a galvanometer mirror in such a way that the laser beam hits the storage phosphor layer to be read out substantially in the form of a point, and passes over this in a linear area. The emission light emitted here by the storage phosphor layer is collected by a photomultiplier, and converted into an electric signal. By conveying the storage phosphor layer perpendicularly to the linear area, it is possible to successively read out individual linear areas of the storage phosphor layer so that finally, a two dimensional X-ray picture is obtained which is made up from individual lines which, in turn, are respectively made up from a number of individual pixels. In order to convert the electric signal of the photomultiplier into individual picture signals allocated to the respective pixels of the X-ray picture, a position reference pulse is produced for each pixel of a line by means of a so-called linear encoder, and the electric signal of the photomultiplier is respectively integrated over a fixed interval of time in accordance with a position reference pulse. The signal value obtained during the respective integration is allocated to the corresponding pixel.
Because the speed of the deflection element is generally liable to fluctuations, the speed of the laser beam is not constant over the whole of the line to be read out, ie. within the predetermined, fixed interval of time, and in accordance with a position reference pulse, the laser beam passes over areas of the storage phosphor layer with different widths, dependent upon the momentary speed. The consequence of this is that, during the described integration of the electric signal of the photomultiplier, areas of different width are generally collected. A high momentary speed of the laser beam thus means that, in the predetermined interval of time, a relatively wide section of the area corresponding to a pixel is scanned by the laser beam. On the other hand, a lower momentary speed leads to the collection of a relatively narrow section of an area of the line allocated to a pixel. Overall therefore, a partial loss of information and so an adverse effect upon the picture quality of the X-ray picture read out has to be accepted.